I switched from the normal hotmail to live mail beta, and the interface is interesting. It doesn't work well with browsers on the Mac. On Internet Explorer on Windows it takes a while to load, but otherwise the interface is interesting. Its more like a normal application. For instance, you have to double click on an email to view it. There is no preview, at least not in the default view. I don't know if there are other views.
The one big feature that I was hoping for was better/easier search, ala gmail. If its their, I haven't seen it. I'll have more once I've used it more.